This invention relates generally to a novel seam tracker apparatus. More particularly, this invention is a seam tracker apparatus including a novel and improved means of biasing the seam tracker probe to a selected mechanical null position.
In the past, it has been common to employ seam trackers which contain a probe which is disposed in a seam or joint to be welded and which responds to displacement of the seam tracker probe in either of two directions, horizontally, or vertically. As is known, this is done as the seam tracker probe moves, for example, within the seam or in abutment to the joint being welded, in order that the seam tracker apparatus may produce an output signal representative of the change of the position of the probe, horizontally or vertically. This output signal or signals is employed to modify accordingly the change of position of the welding head such that the welding head remains properly oriented with the seam or joint to ensure proper welding. While such arrangements have exhibited at least a degree of utility in maintaining the welding head in its proper orientation, room for a significant improvement remains.
A welding apparatus containing a seam tracker generally of the type disclosed in the present application is shown in the patent to Cecil et al., U.S. Pat. No. 3,997,757, issued on Dec. 14, 1976. The seam tracker disclosed therein contains a probe which is connected to a shaft extending through a generally cylindrical seam tracker housing (as disclosed to be of a type illustrated in the patent to Morehead, U.S. Pat. No. 3,171,012). The null point of the probe in the Morehead patent, as described in the Cecil patent is along the longitudinal axis of the seam tracker housing. The means for biasing the shaft to the null point consists of a flange on the shaft which is urged against a seat by a coiled spring disposed between the shaft flange and a collar mounted on the interior portion of the housing. Such an arrangement contains problems both in ensuring that the shaft is urged back to its proper null position (which is both a mechanical and electrical null position) when pressure is removed from the probe, and also in allowing slippage of the flange on a seating collar, such that excessive pressure on the probe may distort the orientation of the flange to the seating collar and produce an improper position of the other end of the shaft which contains a means for sensing the position of that other end of the shaft in order to determine the location of the probe with respect to the null position. Moreover, the pressure of the coil spring at various points around the circumference of the portion of the coiled spring abutting the flange will tend to be irregular, so that return to the null point or urging of the shaft back toward the null point as the position of the seam changes may not be totally accurate. Thus, improper indications of the exact position of the probe with respect to the null position may result.
Many of the shortcomings of the prior art seam tracker were overcome by an apparatus invented by Messrs. William Toohey and Franco Ferrari and described in a co-pending application entitled "Seam Tracker," Ser. No. 582,745, filed 2/23/84 ("Toohey application"), assigned to Mr. Larry Russell. The seam tracker apparatus disclosed in the Toohey application included a shaft on which the probe for following the seam in the workpiece is mounted. The shaft is mounted so that the upper end has two degrees of freedom with a mechanical null point. A first coil spring and a sleeve slidably mounted over the shaft supporting unit urges the shaft downward against a pair of stops to provide one null position in one direction of movement, e.g., along the X axis. A second coil spring is used to bias the shaft to a null position along the other axis. The structure is designed to ensure that the shaft returns to its null position when the probe is removed from contact with the workpiece.
It has been discovered, however, that the sleeve tends to rock on the support unit as a result of uneven pressure thereon by the first coil spring and wear. The rocking action creates friction which distorts the response of the apparatus to changes in the seam in the workpiece being followed.
The problems enumerated in the foregoing are not intended to be exhaustive, but rather are among many which tend to impair the effectiveness of previously known seam trackers. Other noteworthy problems may also exist; however, those presented should be sufficient to indicate that seam trackers appearing in the prior art have not been altogether satisfactory.